1. Field of the Invention
The present invention relates to a magnetic disk apparatus using combination heads in each of which a write head and a read head are arranged at an edge of a rotary actuator and relates to an offset measuring method of such an apparatus. More particularly, the invention relates to a magnetic disk apparatus for accurately measuring an offset caused by a core deviation between the write head and the read head in a default state of apparatus parameters and relates to an offset measuring method of such an apparatus.
2. Description of the Related Arts
As a head which is currently used in a magnetic disk apparatus, a combination head in which a read only MR head and a write only inductive head are arranged is used. The combination head is supported at an edge of an actuator, moved in the radial direction which transverses medium tracks, and positioned. Further, a data surface servo in which servo frames on which position information had been recorded have previously been arranged on a data surface of the medium at predetermined intervals in the direction of tracks is used. Therefore, when data is written, the data is written onto the medium by the inductive head in a state where the center of the MR head is positioned to the track center so as to enter an on-track state by a position signal demodulated from a reproduction signal of the MR head. In the reading mode for reproducing the data written on the medium, in a state where the center of the MR head is positioned to the track center so as to enter an on-track state by the position signal demodulated from the reproduction signal of the MR head, the data is read out from the medium by the MR head and demodulated. Since the write head and the read head are separately arranged on the combination head as mentioned above, a deviation generally occurs between the write center position of the write head at the time of the data writing and the read center position of the read head at the time of the data reading. It is, therefore, necessary to previously measure a deviation amount between the write center position and the read center position and perform a control to position the read head to the write center by offsetting it by such a deviation amount when the data is read out. According to a method of measuring such an offset amount, first, random data is written as a reference signal indicating the position of the write head, after that, the data is read out while the read head is gradually offset, and the center of a range of the offset amount in which an error rate is the minimum or the offset amount in which no error occurs is set to the optimum offset amount.
However, in the measurement of the offset caused by such a separate arrangement of the write head and the read head, even when parameters of a read channel are set as default parameters, the data has to be accurately read out to a certain extent. This is because since the data written on the medium is used for adjustment of the parameters of the read channel, the parameters for adjustment written on the medium have to be able to be read out by the default parameters before adjustment of the parameters. In the conventional read channel, since the number of parameters which need the adjustment by the data read out from the medium is relatively small, an error rate of a certain degree can be assured even in the setting state of default. In recent years, however, owing to the progress of the read channel, even in case of a low S/N ratio, an error rate equivalent to the conventional one can be assured. The setting of parameters, however, becomes fine by an amount of such progress. In the default setting state in which the parameters are not adjusted at all, it is a present situation that no data can be read out. As for the evaluation in the measurement of the offset caused by the separate arrangement of the write head and the read head, it is a general way that a data correcting function by an ECC is used and an offset amount in which one track is read and the number of sectors in which errors that cannot be corrected by the ECC occurred, namely, a sector error rate when one cylinder is read becomes the minimum, or the center of a range of the offset in which uncorrectable errors are not generated is set to the optimum offset amount. This is because the discrimination about whether the correction of the ECC is possible or not can be performed by hardware, and the deterioration of the error rate due to unadjustment of the parameters of the read channel can be compensated by the ECC. However, there is a problem that the evaluation by the error rate of the ECC is weak for a medium defect. That is, it is assumed that at the time of offset measurement, a medium defect locally exists on either one of the sides of the measuring track, for example, on the outer side in the disk radial direction. In this case, when the position of the read head is close to the outer side, a burst error occurs by the medium defect on the outer side and it cannot be corrected by the ECC, so that the error rate deteriorates. Consequently, the normal error rate is evaluated by the ECC in the area on the inner side where there is no defect. The optimum offset amount in this case is deviated to the inner side as compared with the inherent optimum offset amount in the case where there is no medium defect, and it is weak for the medium defect.
According to the invention, there is provided a magnetic disk apparatus in which parameters of a read channel can be easily set at the time of measurement and, even when there is a medium defect or the like, an optimum offset can be accurately obtained in a default setting state and to provide an offset measuring method of such an apparatus.
First, according to the invention, there is provided a magnetic disk apparatus which has combination heads formed by arranging read heads and write heads at an edge of an actuator and in which information is recorded or reproduced while the head is positioned to a target track so as to enter an on-track state on the basis of a position signal of a medium demodulated from a reproduction signal of the read head, further upon reproduction, the reproduction signal from the read head is equalized to a Partial Response waveform, and after that, it is maximum likelihood detected, thereby demodulating information. According to the invention, a measurement pattern writing unit and an optimum offset measuring unit are provided for the magnetic disk apparatus. The measurement pattern writing unit writes random patterns to adjacent tracks locating on both sides of a measuring track whose offset is measured, or writes a preamble pattern. The optimum offset measuring unit obtains the sum xcexa3(yxe2x88x92W)2 of the squares of errors between sampling values y of a Partial Response waveform obtained by equalizing the reproduction signal of the preamble pattern and a predetermined ideal sampling value W in the case where there is no noise while changing an offset amount, calculates a mean per sample [hereinbelow, referred to as a xe2x80x9cmean square error (MSE)xe2x80x9d] by dividing the sum xcexa3(yxe2x88x92W)2 of the square errors by the number of samples (N), and determines an offset amount in which the mean square error MSE becomes the minimum for a change in offset amount as an optimum offset. In case of Partial Response Class 4 maximum likelihood detection (hereinbelow, referred to as a xe2x80x9cPR4MLxe2x80x9d), when assuming that the magnetization inversion is set to bit xe2x80x9c1xe2x80x9d and the non-magnetization inversion is set to bit xe2x80x9c0xe2x80x9d as a preamble pattern, the measurement pattern writing unit writes a preamble pattern xe2x80x9c01010101 . . . xe2x80x9d in which xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d are repeated onto the measuring track. In the offset measurement of the invention, therefore, in case of PR4ML, by using the preamble pattern which is repeated like xe2x80x9c01010101 . . . xe2x80x9d, a waveform passed through a low pass filter is equivalent to the waveform which was correctly equalized to the Partial Response waveform. Consequently, the mean square error as an evaluation amount does not include any equalization error but includes only a power of noises. When the read head is deviated from the track, a signal decreases and noises relatively increase. Therefore, when the read head correctly enters the on-track state, the value of the mean square error becomes the minimum and the offset amount at that time becomes the optimum offset amount. According to the conventional offset measurement, although the number of sectors in one track is equal to about hundreds of sectors, the number of bits corresponding to the number of samples in the invention is equal to about 1e6, namely, about 1xc3x97106 and is remarkably large. The measurement error of the mean square error per sample obtained in the invention is extremely smaller than that of the conventional sector error rate, so that the optimum offset amount can be further accurately obtained.
The optimum offset measuring unit sets a cut-off frequency Fc of a low pass filter which is used for equalization to the Partial Response waveform so as to be substantially the same as a Nyquist frequency fn corresponding to xc2xd of the sampling frequency fs. As a boost amount Fb of the low pass filter which is used for equalization to the Partial Response waveform, the optimum offset measuring unit sets a peak frequency of filter gain characteristics so as to be matched with a peak frequency of medium noises. By this setting, a signal in which the preamble pattern, medium noises, and circuit noises have been synthesized is outputted as an output of a preamplifier. However, since the circuit noises do not contribute to a change in mean square error at the time of changing the offset amount of the head, by matching the peak of the gain characteristics of the filter with the peak of a power spectrum of the medium noises, a ratio between the medium noises and the circuit noises in the signal after the output from the filter is larger than that before the output from the filter. Consequently, the change amount of the mean square error at the time of offsetting the head increases and the sensitivity of the measurement rises.
The optimum offset measuring unit sets a specific tap coefficient of a transversal filter, for example, an FIR filter (finite impulse response filter) which is used for equalization to the PR4 response waveform to a value other than xe2x80x9c0xe2x80x9d and sets the remaining tap coefficients to xe2x80x9c0xe2x80x9d, thereby forming simple low pass filter characteristics and enabling the parameters to be simply set. That is, in the conventional offset measurement using random data, since a feedback control of an AGC or a PLL is performed on the basis of the signal equalized to the PR4 response waveform, it is necessary to adjust the equalizer so that the signal read out from the medium can be accurately equalized. According to the invention, however, by using the preamble pattern (dipulse pattern), such as xe2x80x9c010101010 . . . xe2x80x9d in which xe2x80x9c1xe2x80x9d is written every two bits, even if a simple low pas filter is used as an initial setting of the equalizer, the same equalization waveform as a PR4 response waveform is outputted and the feedback control of the AGC or PLL is correctly performed, so that the parameters can be easily adjusted.
Prior to the measurement of the mean square error per sample, the optimum offset measuring unit automatically adjusts an automatic control gain of a variable gain amplifier that is used for equalization to the Partial Response waveform and a timing of a PLL circuit (timing recovery circuit) for extracting a read clock by the training due to the reproduction signal of the preamble pattern. Prior to the measurement of the mean square error per sample, the optimum offset measuring unit calculates a mean square error every data frame or data sector on the track in a state where an offset of the head is fixed to a predetermined offset amount over the measuring track, compares it with that of the other frame or sector. When the mean square error is extremely large, such a frame or sector is regarded as a frame or sector having a medium defect and is excluded from the measuring target. In this manner, the mean square error is measured every sector or every frame instead of calculating the mean square error in all of the bits in one cylinder. It is regarded that the sector or frame having the remarkably large mean square error has a medium defect. The influence by the medium defect can be completely eliminated by using the mean in which the values of the units are excluded. The optimum offset measuring unit calculates the mean square error for a plurality of measuring areas where measuring cylinders are uniformly distributed and arranged in the circumferential direction, thereby reducing an influence of a periodic eccentric disturbance such as medium eccentricity or the like. The optimum offset measuring unit measures the optimum offset amounts based on the measurement of the mean square error per sample on at least two tracks on the inner side and outer side of the medium and derives the optimum offsets with respect to the other tracks by a linear interpolating calculation. As mentioned above, by reducing the number of measuring tracks to the necessary minimum limit, a process burden on the offset measurement can be reduced. As measuring tracks, three tracks such as inner track, center track, and outer track are desirable. Although the offset changes by a sine function for a change in yaw angle xcex8y, since a range of the yaw angle is relatively narrower than an arm length of the rotary actuator, it can be sufficiently approximated by the linear interpolation. When a plurality of combination heads are provided, the measurement pattern writing unit and optimum offset measuring unit obtain the optimum offset amount based on the measurement of the mean square error per sample every combination head.
According to the invention, there is also provided an offset measuring method of a magnetic disk apparatus which has combination heads formed by arranging read heads and write heads at an edge of an actuator and in which information is recorded or reproduced while the head is moved to a target track so as to enter an on-track state on the basis of a position signal of a medium demodulated from a reproduction signal of the read head, further at the time of reproduction, the reproduction signal from the read head is Partial Response equalized and, after that, maximum likelihood detected, thereby demodulating information. The measuring method comprises:
a measurement pattern writing step of writing random patterns to adjacent tracks locating on both sides of a measuring track to measure an offset or writing a preamble pattern; and
an optimum offset measuring step of obtaining the sum of squares of errors between sampling values of the Partial Response waveform obtained by equalizing the reproduction signal of the preamble pattern and a predetermined ideal sampling value in the case where there is no noise while changing an offset amount of the combination head, subsequently calculating a mean square error per sample by dividing the sum of the square errors by the number of samples, and determining an offset amount in which the mean square error is the minimum for a change in offset amount as an optimum offset.
The details of the offset measuring method are the same as those in the case of the apparatus.
The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description with reference to the drawings.